U.S. Pat. No. 4,046,847 issued Sept. 6, 1977 discloses a method for rendering zinc oxide varistors stable for AC operation. U.S. patent application Ser. No. 967,196, filed Dec. 7, 1978, now U.S. Pat. No. 4,243,622, issued Jan. 6, 1981, discloses a method for rendering a zinc oxide varistor stable by means of a single heat treatment application.
U.S. patent application Ser. No. 161,935, filed June 23, 1980, now abandoned, discloses the use of an insulating glass collar around the periphery of zinc oxide varistors to prevent the varistors from becoming unstable in the presence of a nonoxidizing gas.
U.S. Pat. No. 3,959,543, issued May 25, 1976 describes a specific glass composition for providing an insulating collar to zinc oxide varistors.
Aforementioned U.S. Pat. No. 4,046,847 describes the instability problems that occur when zinc oxide varistors are used without a post sinter heat treating process. The instability is caused by changes in the "bulk" conductivity through the bulk region of the disc when the disc is used in an AC voltage application. When the disc is used in a DC voltage application it is found that "bulk" instability occurs to some extent whereas, "rim" instability occurs to a much greater extent. When the varistor is subjected to a source of DC voltage, after heat treating the varistor as described in the aforementioned U.S. Pat. No. 4,046,847, the bulk region of the disc remains relatively stable whereas the rim region of the disc rapidly becomes unstable. For purposes of this disclosure "rim" instability is defined as the instability that occurs in the region of the vicinity of the varistor rim whereas "bulk" instability occurs in the remaining region through the varistor.
Varistors having glass rims are found to be limited to a particular voltage level above which the insulating properties of the glass are insufficient to prevent flashover from occurring between opposite electrode faces of the varistor. A coating of an inorganic resin or ceramic material is therefore required to make the varistors suitable for high voltage applications. However, when the organic resin or ceramic material is heated above a specified temperature to cure the resin or set the ceramic, the high voltage discs become unstable when subjected to DC voltages.
The purpose of this invention is to describe methods and materials for rendering high voltage resistors stable under DC voltage conditions.